The present invention generally relates to a photo-voltaic system and, more particularly, to a charging and discharge control circuit utilizable in said photo-voltaic system for controlling a charging current as well as a discharging current by the detection of increase in voltage stored in a storage battery.
The charging and discharge control circuitry currently employed in photo-voltaic systems can be classified into two systems; a voltage control system and a current control system. The voltage control system is operable to detect a high voltage in the storage battery for the charging control and to detect a low voltage for the discharge control and is widely used in practice because the required circuitry is simple and requires no maintenance.
However, when and so long as the storage battery is charged or discharged, the voltage is greatly affected by a current then flowing and, accordingly, changes considerably. By way of example, the temperature dependent characteristics of the charging and discharge voltages are such as shown in FIG. 2. The temperature dependent characteristics of the charging voltage at 25.degree. C. and 5.degree. C., are shown by solid-lined and chain-lined curves, respectively, both generally exhibiting the increase in voltage with time, the rate of increase being so steep at about 100% of capacity that it can be easily detected. On the other hand, the temperature dependent characteristics of the discharge at 25.degree. C. and 5.degree. C. are shown by by solid-lined and chain-lined curves, respectively, both generally exhibiting the decrease in voltage with time, and since the voltage changes to a great extent at, for example, about 50% of capacity while considerably affected by temperature, it cannot be easily detected. In the graph of FIG. 2, the single-dotted chain line represents a preset level for the high voltage.
As discussed above, with the voltage control system, there is a problem in that the discharging state of the storage battery cannot be precisely detected and, therefore, the storage battery tends to be excessively discharged. The excessive voltage discharge depreciates the service life of the storage battery and, therefore, countermeasures must be employed for avoiding the problem.
On the other hand, the current control system is such that, when the integrated value of the charging or discharging current which is integrated subsequent to the complete (100%) charging of the storage battery falls below a predetermined value, the discharge control (interruption of the discharge) is effected, but when it exceeds 100%, the charging control (interruption of the charging) is effected. This system, although effective to precisely detect the charging and discharge state of the storage battery for a limited period of use, has a problem in that, as a result of lowering of the voltage storage capability of the storage battery consequent to the self-discharge occurring in the storage battery or the generation of gases attributable to decomposition of components of the storage battery, both occurring as the storage battery is used for a long period of time, an integrator-control circuit tends to produce a great error between the amouut of voltage required to be stored and the amouut of voltage actually stored. Because of this, manual interventions are required to permit the storage battery to be 100% charged and to form a reference condition required for a control circuit to be reset.